Indeed, as I mentioned the Sunrise Corridor thing was done by a group that did not include Disney. Disney did not get to make a choice yet about whether their land would be used or not. The original land offered became nonviable with the reroute, and Disney was not party to that decision. As for whether Disney will or will not be involved in deciding the site of the South I-Drive station remains to be seen. But that being part of the Sunshine Corridor now it is not just Disney and Brightline's decision to make. The Sunshine Corridor Consortium to use an informal name for the collective is the one that has to come to an agreement on it.
You are quite correct in speculating that Universal through the Sunshine Group may be playing its cards in the background to ensure that the station is not on Disney land. We'll see how all that unfolds.
BTW, since the current proposed station is at South I-Drive, my assumption is that it will be close to South I-Drive, which to me suggests that the tentative location proposed is between World Center Drive and Osceola Parkway on the west of I-4 and east of I-Drive. It could be further South, but then it would have nothing to do with I-Drive.